VIENNA —
Six world powers and Iran began “substantive” talks on Tuesday in pursuit of a final settlement on Tehran's contested nuclear program in the coming months despite caveats from both sides that a breakthrough deal may prove impossible.

Senior U.S. and Iranian officials met separately for 80 minutes on the sidelines of the negotiations in Vienna.

Details were not given, but such bilateral talks were inconceivable before the 2013 election of Hassan Rouhani, a relative moderate, as president of Iran. U.S.-Iranian dialogue is seen as crucial to any breakthrough nuclear agreement.

“The conversation was productive and focused mainly on how the comprehensive talks will proceed from here,” a senior U.S. State Department official said on condition of anonymity after Under Secretary of State for Political Affairs Wendy Sherman's meeting with Iranian Deputy Foreign Minister Abbas Araqchi.

Sherman headed the U.S. delegation, while Iranian Foreign Minister Mohammad Javad Zarif and Araqchi led Tehran's negotiating team at the table with Britain, China, France, Germany, Russia and the United States.

After Sherman's meeting with Araqchi, the Iranians met on Tuesday evening with all six powers to continue talks on how to approach future negotiations, diplomats said. The discussions will resume on Wednesday.

“Much of the first day was focused on discussions about process for how the comprehensive talks will proceed,” a senior U.S. official said. “We made clear that every issue is on the table as part of the comprehensive negotiations, and now it's time to dig into the details and get to work.”

In the evening session between Iran and the six “substantive issues began to be discussed”, the U.S. official added.

It is the first round of high-level negotiations since a Nov. 24 interim deal that, halting a decade-long slide towards outright conflict, has seen Tehran curb some nuclear activities for six months in return for limited relief from sanctions to allow time for a long-term agreement to be hammered out.

The stakes are huge. If successful, the negotiations could help defuse many years of hostility between Iran - an energy-exporting giant - and the West, ease the danger of a new war in the Middle East, transform power relationships in the region and open up vast new possibilities for Western businesses.

The talks are expected to last two or three days.

Modest Expectations

Araqchi sounded upbeat about the initial 40-minute discussions but appeared to draw a line against Tehran's ballistic missile program being addressed in any future talks.

“We had good discussions ... and we are trying to set an agenda. If we can agree on an agenda in the next two to three days, it means we have taken the first step. And we will move forward based on that agenda,” he said. “This agenda ... will be about Iran's nuclear program and nothing else, nothing except Iran's nuclear activities can be discussed.”

He was answering a question about Iran's ballistic missile work after U.S. officials said they want Tehran to accept limitations on any nuclear-capable missile technology as part of any long-term deal reached by Iran and the powers.

There may be other sticking points in the talks. Iran says it will not cede its “right” to install advanced centrifuges to refine uranium, signaling defiance in a manner that may irk the United States and its European allies.